Naming organic compounds can be a challenge to any chemist at any level. Historically, chemists developed names for new compounds without any systematic guidelines. In this century, the need for standardization was recognized. For simple molecules, the nomenclature system worked out by the International Union of Pure and Applied Chemists (IUPAC) works well. For complex molecules, the IUPAC names are so long that no one in their right mind would use them. The net result is that a hodgepodge of IUPAC names and historic or common names are used. Any one compound may have five or six different names. So, what we want to accomplish in this module is simply to establish the fundamentals of the IUPAC system and apply them to naming alkanes, alkenes and alkynes. These groups are hydrocarbons, compounds made of the elements carbon and hydrogen.
Numerical Prefixes = Number of Backbone Carbon Atoms
The prefix in the name of an organic molecule indicates the number of carbon atoms found in the longest continuous chain of carbon atoms in the molecule. You need to memorize the following prefixes:
Prefix # C atoms
Alkanes = -ane ending
The alkanes are the least complex hydrocarbons. The alkane family uses the prefix for the number of carbons and an -ane ending. An alkane can be recognized by its general formula, CnH2n+2, where n is the number of carbon atoms in the compound. For example, C5H12 has five carbon atoms pentane. Each member of the alkane family differs from the next by a — CH2 — group, and all the carbons are connected by single bonds.
Name the following compounds:
b. C2H6 or CH3CH3
c. C3H8 or CH3CH2CH3
d. C4H10 or CH3 CH2CH2CH3
All of the formulas fit into general formula, CnH2n+2, therefore the bonds in these compounds are single bonds; they are alkanes. Use the numerical prefix for the number of carbon atoms with the -ane ending. a. one C atom = methane
b. two C atoms = ethane
c. three C atoms = propane
d. four C atoms = butane
Alkenes = -ene ending
Hydrocarbons that contain multiple bonds are called unsaturated hydrocarbons. If the hydrocarbon has one double bond, its general formula will be CnH2n, where n is the number of carbon atoms in the compound. The alkene family uses the -ene ending. The double bond is stronger than a single bond, and the bond length between the carbon atoms is shorter in the double bond. It is also more reactive than a single bond since the π bond (the second pair of electrons) is farther from the nuclei.
Naming is a little bit more complex for alkenes than alkanes. Since the double bond could appear at various sites in a typical molecule, we have to specify where it is. To do so, number the carbon backbone so that the lowest possible number is used to describe the double bond position. The lowest number of the two C atoms involved in the double bond is used in front of the name to indicate the C=C position. The number is place at the beginning of the name and is separated with a dash.
In the expanded structure formulas shown below, it is understood that since H only forms one bond, any double bonds are between carbon atoms. The expanded structures give a bit more information about how many H atoms are attached to each C atom. Example 2:
Name the following compounds.
a. C2H4 or H2C=CH2
b. C3H6 or CH3CH=CH2
c. C4H8 or H2C=CHCH2CH3
d. C4H8 or CH3CH2=CH2CH3
e. C5H10 or CH3CH2CH2CH=CH2
a. 2 C atoms = ethene (since there are no options for the position of the C=C, we do not need to...